Various processes for the manufacture of microspheres of small diameter in the range 40-60 micrometers and with somewhat larger microspheres having a diameter in the range 0.25 to 5.0 millimeters have been described in the prior art, for example Japanese Published Patent Application Number 57-84731 and U.S. Pat. No. 4,746,468.
U.S. Pat. Nos. 3,491,492, 4,068,718, 4,427,068 and 4,713,203 all describe processes for manufacture from clay or bauxite materials substantially spherical ceramic particles or pellets in the size range 0.25-5.0 millimeters, primarily for use as proppants. Apart from U.S. Pat. No. 4,713,203 which utilizes naturally occurring bauxite fines as feedstock the other prior art documents describe the use of relatively coarse clay or bauxite particles. U.S. Pat. No. 4,427,068 does imply however that expensive grinding of calcined clay or bauxite may be employed to produce particle size less than 15 micrometers.
Each of U.S. Pat. Nos. 3,491,492, 4,068,718, 4,427,068 and 4,713,203 is concerned with the manufacture of proppants for hydraulic fracturing of subterranean formations and each requires the physical formation of pellets by agglomeration in a rotary pelletizer or the like, with or without a binder. Subsequently calcining of the green pellets is usually carried out in a rotary calcining kiln.
Previously attempts to produce spherical proppants by spray drying have produced rounded non spherical particles characterized by a hollow recess similar in appearance to a mushroom cap. This shape has been ascribed to aerodynamic deformation of the slurry droplet in the hot gas stream before the particle dried.
Metal-matrix composites (MMCs) consist of a matrix of a metal or alloy into which one or more second phases have been incorporated with the aim of reinforcing for improved properties. After many years of research, these materials are now becoming commercially available. The enhancement in properties over those of the matrix material which can be achieved include:
improvement in strength at ambient and high temperatures, PA1 improvement in stiffness, PA1 improvement in fatigue strength, PA1 improvement in wear resistance, PA1 reduction in coefficient of thermal expansion. PA1 matrix metal or alloy, PA1 content of the reinforcing phases, PA1 chemistry, geometry, distribution and orientation of the reinforcing phases, PA1 nature of the interface between the reinforcing phases and the matrix material, PA1 manufacturing method, PA1 thermo-mechanical history. PA1 preparing a dispersion of bauxite or bauxitic clay; PA1 classifying the dispersed bauxite particles to recover the ultrafine fraction; PA1 adding small quantities of water soluble salts, mineral compositions or organometallic complexes to control the microsphere surface chemistry so as to enhance the wetting and dispersion of the microsphere and improve its ability to bonding strongly with the matrix materials in use; PA1 spray drying the dispersion to produce green microspheres of a predetermined means particle diameter, and PA1 subjecting said green microspheres to calcination and sintering to produce microspheres having a size within the range 0-100 micrometers, preferably from 1 to 50 micrometers and most preferably less than 30 micrometers, said microsphere being characterised by a substantially solid form having a pycnometric density substantially falling in the range 3.2 to 3.9 g/cm.sup.3, a BET surface area substantially falling in the range 0.05 to 0.5 m.sup.2 /g and a crystal grain size less than 4 micrometres. PA1 the need for expensive pregrinding is eliminated PA1 the need for expensive precalcination is eliminated PA1 high strength of the green microsphere is obtained without the addition of a binder PA1 the very high surface area of the feed material makes it highly reactive. This leads to a reduction in sintering time, hence in energy consumption PA1 an exceptionally high degree of uniformity in the
These property enhancements have made MMCs attractive for structural applications at room temperature and at elevated temperatures. The magnitude of the enhancements in properties depend on many factors, such as, the:
In addition, all other factors which generally affect properties of metallic materials, such as the heat-treatment, fabrication, porosity level, etc., also affect the properties of MMCs.
Aluminium, magnesium, titanium, copper and their alloys can be used as matrix materials. For reinforcing purposes, a variety of compounds such as carbides, oxides, nitrides, borides, etc. and elements such as carbon and boron, in various forms such as fibres, whiskers, platelets and particulates, have proven to be effective. High performance fibres and whiskers are manufactured by costly, energy-intensive processes which make them expensive, resulting in high cost to the finished MMC product. There are certain lower performance fibres and particulates, however, which produce moderate property improvements at low cost. The present invention relates to a reinforcement for low-cost MMCs incorporating an in inexpensive naturally occurring raw material.
Although, some naturally occuring materials such as natural graphite, mica and zircon sand have been examined for reinforcing application in MMCs, they are not recommended because the properties of the resulting MMCs are generally poorer than those of the matrix. An important aspect of this invention is the use of a naturally occuring mineral as the starting material for reinforcement of MMCs and other composites leading the enhancement in properties.
The geometries of reinforcing materials employed by MMC producers are typically fibre, platelet and particulate. The fibre geometry has a long dimension in one direction which is the preferred direction of reinforcement. Little or no reinforcing effect, sometimes even deleterious effects, are observed in the directions perpendicular to the longitudinal direction. The platelet geometry, in turn, imparts reinforcing effects only in the plane of the platelet which, again, is the preferred plane for reinforcement, Consequently, MMCs reinforced by fibres and platelets exhibit anisotropic properties. Randomly oriented fibres and platelets can restore isotropy but this is difficult to achieve in practice and unlikely to be maintained during forming operations.
Particle reinforced composites do not exhibit the anisotropic characteristics of fibre or platelet composites. Hence they are the most efficient for the production of isotropic MMCs, in addition to being low in cost. The use of the ceramic microspheres defined above as a reinforcing material offers, by virtue of its geometry, a high degree of isotropy to the MMCs.
The particulate materials presently used for reinforcing purposes have angular or irregular shapes. When reinforcements of such shapes are incorporated in a metallic matrix, the particles can act as stress raisers under the action of an applied stress. This can lead to premature nucleation of cracks with associated reduction in plasticity of the MMC. Reduced plasticity is reflected in lower elongation, toughness, formability and perhaps strength. The geometry and fine microstructure of the particulate material embodied in this invention is free of any stress raising features such that MMCs reinforced with the material have improved ability for plastic deformation.
Microspheres of the above type have now been determined to be suited for use as a reinforcement material in metals to form metal matrix composites as well as in other materials such as plastics, to provide improved strength and wearing properties. In making microspheres for this purpose it is important to ensure that the microspheres have properties which promote "wetting" which determines the extend to which the interface of the microsphere transforms the strengthening effect of the ceramic.
Good wettability between the matrix alloy and the reinforcing material is critical for the production of good quality MMCs. Many studies have shown that commercial reinforcing materials are not easily wetted by molten metals and alloys, and similar comments apply with equal validity to offer materials such as plastics. A good reinforcing material, therefore, should exhibit good wettability while being relatively inert to the matrix alloy. Such a reinforcing material is provided in this invention.